1. Field of the Invention
The invention relates to the field of ice detection systems for aircraft and, in particular, to an ice detection system for low observable aircraft.
2. Description of Related Art
Aircraft icing can occur under certain atmospheric conditions. The icing primarily forms on the leading edge of the wings. Such ice accretion, if allowed to build up, can cause a loss of lift, which can, in extreme cases, cause the aircraft to crash. Thus modern commercial aircraft incorporated anti-icing devices. For example, large aircraft incorporate hot air ducts along the leading edges of wings. Hot bleed air from the compressor stages of the turbine engines are fed through these ducts, melting the ice. Smaller aircraft use inflatable boots that can be pulsed to expand and contract, breaking up the ice. Other systems involve the use of electromechanical actuators that flex the outer skin of the wings breaking up the ice. On most small aircraft, de-icing systems are not employed. Therefore, the pilot is required to fly the aircraft out of the “ice forming” environment. All aircraft having de-icing systems must have ice formation sensors strategically placed to sense the ice forming so that the de-icing system can be actuated in a timely manner.
Even on aircraft that do not have de-icing systems, detection systems are often incorporated. The most obvious method is visual examination by the flight crew. While the pilot can usually see the wings on small general aviation aircraft, on larger aircraft the wings are not always visible from the flight station. At night, visual examination may not be possible. Thus an ice detection system will give the pilot warning and allow him or her to fly the aircraft out of the area. If the aircraft is unmanned, the remote operator will have the same capability.
There are numerous types of ice detection systems available, for example, U.S. Pat. No. 6,052,056 “Substance Detection System” by J. D. Burns, et al. In this system a modulated light source is directed to an optical sensor located in an area where ice will tend to accumulate, such as an aerodynamic surface or engine inlet. The sensor transmits light back to a detector that is proportional to the amount of ice on the surface.
Another approach is to use ultrasonic transducer guided waves, which are applied to the aircraft's skin. The waves interact with the reflective geometry of the skin and a portion of the waves scatter back to the transducer. By monitoring the amplitude, frequency change, etc. of the returned waves, ice build up on the skin can be identified. Magnetostrictive ice detection is also available. Sensors vibrate ultrasonically at a set frequency. As ice accretes on a probe, the vibrational frequency decreases, which is detected signaling the to remove the ice. In another system a detector operates periodically by heating a temperature element to a constant temperature. A microprocessor measures the element's rate of temperature increase by comparing the time it takes the element to pass through two reference temperatures. Since the melting process absorbs considerable energy, the temperature increases at a slower rate when ice has accumulated.
Another approach is to use capacitance probes mounted on the external surface. Examples of these can be found in U.S. Pat. No.: 4,766,369 “Ice Detector System” by L. M. Weinstein; U.S. Pat. No. 45,569,850 “Ice Detector” by R. L. Raunchorst, 111; and 5,854,672 “Apparatus And Method For Determining The Existence Of Ice Or Water On a Surface From The Capacitance Between Electrodes On Said Surface” by J. J. Gerardi, et al. In these devices, capacitance probes, generally spaced conductive electrodes encapsulated in a non-conductive substrate, are mounted on a surface where ice will tend to accumulate. The accumulating ice, of course, will change the capacitance of the probe, which can be sensed, by a capacitance measuring circuit.
The problem with existing capacitance probe type ice detection systems is that they do not lend themselves to use on low radar observable aircraft. The lead wires to the probes and the probes themselves tend to increase the radar signature on the aircraft. The conductive probes and lead wires scatter the incoming radar signals and have radar cross-sections that are much to large for low observable aircraft applications.
Thus, it is a primary object of the subject invention to provide an ice detection system for an aircraft.
It is a still further object of the subject invention to provide an ice detection system for an aircraft having a low radar cross-section.
An additional object of the subject invention is to provide an ice detection system using capacitance probes for an aircraft having a low radar cross-section.